The present invention relates to a super-precision positioning system and more particularly to a super-precision positioning system which realizes a move of long stroke and a high rigidity.
With the recent advancement of precision of super-precision machine tools, semiconductor related equipment and measuring instruments, it has been strongly desired to improve performance of a super-precision positioning system and a super-precision feeding system which are important components of those equipment. A piezoelectric element is often used as an actuator in the system which requires high rigidity, high resolution and the like among those systems. However, the piezoelectric element has a disadvantage that its displacement is very small. In order to make up for such disadvantage, a spanworm method, a method combined with Walking Drive and a coarse actuator, a method utilizing rapid deformation of the piezoelectric element and the like have been proposed. Although these methods allow the piezoelectric element to be driven for a long stroke, it cannot be said that the rigidity of the piezoelectric element is fully utilized from the aspect of the rigidity when it is used in a positioning system.
The circumstances around this point will be explained by exemplifying the spanworm method. FIG. 17 shows the driving principle of the spanworm method which has been implemented since the past and FIG. 18 shows patterns of driving voltages applied to each piezoelectric element used in the spanworm method. In FIG. 17, a pair of feeding elements 3 are secured to a pair of supporting sections 2 provided at spatial positions which face each other while interposing a moving body 1 therebetween so that the center the elements 3 is supported by the supporting sections 2. One end of pairs of clamp elements 4a and 4b are secured to both ends of the pair of feeding elements 3 such that the other end of the clamp elements are opened to be extendible.
Next, the operation of the spanworm method will be explained based on FIG. 18. Numbers marked at the upper part of FIG. 18 represent timings. Each timing of 1 through 6 corresponds to steps 1) through 6) in FIG. 17. The horizontal axis represents a phase (it is shown as 2.sup.1 radians until when one period of the operation of the spanworm method is completed and the vertical axis represents a voltage value to be applied to each piezoelectric element. FIG. 18 shows each voltage pattern of the clamping element 4a, the feeding element 3 and the clamping element 4b. At first, at Timing 1, the clamping element 4a is released and the clamping element 4b is clamping the moving body 1. Next, the feeding element 3 extends while clamping the moving body 1 by the clamping element 4b between Timing 1 and Timing 2. Accordingly, the moving body 1 is driven in the right direction. At Timing 2, a stroke of the feeding element 3 reaches its limit and the clamping element 4a clamps the moving body 1 at Timing 3. Then, the clamping element 4b releases the clamp at Timing 4. The feeding element 3 contracts to the original state in order to restore the stroke between Timing 4 and Timing 5. During this time, the moving body 1 is driven in the right direction further. At Timing 5, the contraction of the feeding element 3 reaches to its limit and the clamping element 4b clamps the moving body 1. After that, the clamping element 4a releases the clamp at Timing 6, thus returning to Timing 1. The moving body 1 is driven in the right direction for a long stroke by repeating such operations, though it is not continuous operation.
However, while at least either one of the clamping elements always clamps the moving body 1 to support the moving body 1 during the operation of one such period, the clamping elements 4a and 4b clamp the moving body 1 in the same time only at small periods of time between Timing 3 and Timing 4 and Timing 5 and Timing 6. Accordingly, seeing the phase of the clamping elements after positioning, one or two clamping elements clamp the moving body 1 and an enough rigidity may not be realized even though a highly rigid piezoelectric element is used. While the rigidity increases almost proportionally with an increase in the number of the clamping elements clamping the moving body 1, the rigidity is cut almost into a half in case of the above-mentioned spanworm method because one clamping element clamps the moving body 1 most of the time as compared to a case when two clamping elements clamp the moving body 1.
Accordingly, it is an object of the present invention to solve the above-mentioned prior art problem by providing a super-precision positioning system which allows driving in a long stroke, which is highly rigid and which can position at super precision.